zlacker

Is the research design capable of distinguishing from the opposite causation here; what if people who are more depressed are more likely to use facebook more?

This occurred to me because I more and more think of social media use in terms of addiction. For more typical addictive behavior with drugs, we are more likely to think people who are depressed are more likely to develop addictive relationship to alcohol (or other drugs), than we are to think using alcohol (or other drugs) too much will makes you depressed. Although I suppose it can be somewhat circular and complex.

>>jrochk+(OP)
Yes, the study design is able to tell which way the causality points. Not all colleges got access to Facebook at the same time (back before it was open to the general public) so this is sort of a natural experiment: you can look at the colleges that had Facebook access and compare them to the ones that didn't, assuming that they're probably pretty similar in all other confounding variables, and that people don't choose their college based on whether or not it has Facebook access. For more information on this type of design, the phrase to google is "difference in differences".

https://en.wikipedia.org/wiki/Difference_in_differences